Amino acids are fundamental building blocks supporting life. Their role in protein synthesis is well defined, but they contribute to a host of other intracellular metabolic pathways, including ATP ...generation, nucleotide synthesis, and redox balance, to support cellular and organismal function. Immune cells critically depend on such pathways to acquire energy and biomass and to reprogram their metabolism upon activation to support growth, proliferation, and effector functions. Amino acid metabolism plays a key role in this metabolic rewiring, and it supports various immune cell functions beyond increased protein synthesis. Here, we review the mechanisms by which amino acid metabolism promotes immune cell function, and how these processes could be targeted to improve immunity in pathological conditions.
In this Review, Kelly and Pearce highlight the role of amino acids in immune cell function. They outline that beyond driving protein synthesis, amino acids support immunity by modulating immune cell energy metabolism, redox balance, epigenetic modification, nucleotide synthesis, autophagy, and post-translational protein modification.
Macrophage activation status is intrinsically linked to metabolic remodeling. Macrophages stimulated by interleukin 4 (IL-4) to become alternatively (or, M2) activated increase fatty acid oxidation ...and oxidative phosphorylation; these metabolic changes are critical for M2 activation. Enhanced glucose utilization is also characteristic of the M2 metabolic signature. Here, we found that increased glucose utilization is essential for M2 activation. Increased glucose metabolism in IL-4-stimulated macrophages required the activation of the mTORC2 pathway, and loss of mTORC2 in macrophages suppressed tumor growth and decreased immunity to a parasitic nematode. Macrophage colony stimulating factor (M-CSF) was implicated as a contributing upstream activator of mTORC2 in a pathway that involved PI3K and AKT. mTORC2 operated in parallel with the IL-4Rα-Stat6 pathway to facilitate increased glycolysis during M2 activation via the induction of the transcription factor IRF4. IRF4 expression required both mTORC2 and Stat6 pathways, providing an underlying mechanism to explain how glucose utilization is increased to support M2 activation.
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•IL-4 and CSF-1 promote glucose metabolism during M2, or M(IL-4), macrophage activation.•IL-4 and CSF-1 signal via mTORC2 and IRF4 to induce changes in glucose metabolism•Glucose metabolism supports fatty acid synthesis and oxidation in M2 macrophages•mTORC2- and IRF4-dependent changes in glucose metabolism are critical for M2 activation
IL-4 activates macrophages to play a role in immunity to helminths, wound healing, and metabolic homeostasis, but also in cancer progression. Huang et al. identify mTORC2 signaling upstream of IRF4 expression as a critical mediator of changes in glucose metabolism that are essential for IL-4-induced activation.
Vaccination affords protection from disease by activating pathogen-specific immune cells and facilitating the development of persistent immunologic memory toward the vaccine-specific pathogen. ...Current vaccine regimens are often based on the efficiency of the acute immune response, and not necessarily on the generation of memory cells, in part because the mechanisms underlying the development of efficient immune memory remain incompletely understood. This Review describes recent advances in defining memory T cell metabolism and how metabolism of these cells might be altered in patients affected by mitochondrial diseases or metabolic syndrome, who show higher susceptibility to recurrent infections and higher rates of vaccine failure. It discusses how this new understanding could add to the way we think about immunologic memory, vaccine development, and cancer immunotherapy.
Failure of T cells to protect against cancer is thought to result from lack of antigen recognition, chronic activation, and/or suppression by other cells. Using a mouse sarcoma model, we show that ...glucose consumption by tumors metabolically restricts T cells, leading to their dampened mTOR activity, glycolytic capacity, and IFN-γ production, thereby allowing tumor progression. We show that enhancing glycolysis in an antigenic “regressor” tumor is sufficient to override the protective ability of T cells to control tumor growth. We also show that checkpoint blockade antibodies against CTLA-4, PD-1, and PD-L1, which are used clinically, restore glucose in tumor microenvironment, permitting T cell glycolysis and IFN-γ production. Furthermore, we found that blocking PD-L1 directly on tumors dampens glycolysis by inhibiting mTOR activity and decreasing expression of glycolysis enzymes, reflecting a role for PD-L1 in tumor glucose utilization. Our results establish that tumor-imposed metabolic restrictions can mediate T cell hyporesponsiveness during cancer.
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•Tumor cells and TILs compete for glucose within the tumor niche•Metabolic competition can drive cancer progression•Checkpoint blockade antibodies alter the metabolic balance in a tumor•PD-L1 promotes Akt/mTOR activation and glycolysis in tumor cells
Glucose consumption by antigenic tumors can metabolically restrict T cells, directly dampening their effector function and allowing tumor progression. Checkpoint blockade therapy may correct this resource imbalance through a direct effect in the tumor cells.
At the centre of the therapeutic dilemma posed by cancer is the question of how to develop more effective treatments that discriminate between normal and cancerous tissues. Decades of research have ...shown us that universally applicable principles are rare, but two well-accepted concepts have emerged: first, that malignant transformation goes hand in hand with distinct changes in cellular metabolism; second, that the immune system is critical for tumour control and clearance. Unifying our understanding of tumour metabolism with immune cell function may prove to be a powerful approach in the development of more effective cancer therapies. Here, we explore how nutrient availability in the tumour microenvironment shapes immune responses and identify areas of intervention to modulate the metabolic constraints placed on immune cells in this setting.
Many tumors and activated immune cells have high glycolytic activity and accumulate intracellular and potentially extracellular lactic acid. Besides acidifying the intratumoral pH, this might lead to ...an increased lactate concentration in the tumor microenvironment.An acidic pH and high lactate concentrations affect tumor and immune cell function. A low pH impairs the metabolic activity, proliferation, and cytokine production of human and mouse CD8+ T cells, while it can promote melanoma metastasis formation. Neutralizing intratumoral acidification might improve antitumor immunity and attenuate cancer metastasis.Novel findings from in vivo and in vitro isotope tracing establish lactate as a physiological carbon source fueling the tricarboxylic acid cycle in human and mouse cancer cells, mouse CD8+ T cells, regulatory T cells, and anti-inflammatory bone-marrow-derived macrophages.The intact function of lactate dehydrogenase in mouse CD8+ T cells is required for their antigen-specific expansion, cytokine production, and cytotoxicity in vivo.We posit that future efforts to improve the antitumor immune response should overcome the harmful effects of an acidic pH on CD8+ T cells while sustaining their physiological use of lactate as a metabolic fuel.
Recent studies establish that the metabolic byproduct of glycolysis, lactate, is a physiological carbon source for both cancer and immune cells, including CD8+ T cells. Maintaining physiological lactate metabolism in antitumor CD8+ T cells is essential for their cytotoxic activity, while improving their resistance to acidic pH in the tumor microenvironment might be a beneficial candidate approach to cancer immunotherapy.
Lactic acid production has been regarded as a mechanism by which malignant cells escape immunosurveillance. Recent technological advances in mass spectrometry and the use of cell culture media with a physiological nutrient composition have shed new light on the role of lactic acid and its conjugate lactate in the tumor microenvironment. Here, we review novel work identifying lactate as a physiological carbon source for mammalian tumors and immune cells. We highlight evidence that its use as a substrate is distinct from the immunosuppressive acidification of the extracellular milieu by lactic acid protons. Together, data suggest that neutralizing the effects of intratumoral acidity while maintaining physiological lactate metabolism in cytotoxic CD8+ T cells should be pursued to boost anti-tumor immunity.
A “switch” from oxidative phosphorylation (OXPHOS) to aerobic glycolysis is a hallmark of T cell activation and is thought to be required to meet the metabolic demands of proliferation. However, why ...proliferating cells adopt this less efficient metabolism, especially in an oxygen-replete environment, remains incompletely understood. We show here that aerobic glycolysis is specifically required for effector function in T cells but that this pathway is not necessary for proliferation or survival. When activated T cells are provided with costimulation and growth factors but are blocked from engaging glycolysis, their ability to produce IFN-γ is markedly compromised. This defect is translational and is regulated by the binding of the glycolysis enzyme GAPDH to AU-rich elements within the 3′ UTR of IFN-γ mRNA. GAPDH, by engaging/disengaging glycolysis and through fluctuations in its expression, controls effector cytokine production. Thus, aerobic glycolysis is a metabolically regulated signaling mechanism needed to control cellular function.
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•T cells do not require aerobic glycolysis to fuel proliferation or survival•Glycolysis is specifically required for effector cytokine production in T cells•When not engaged in glycolysis, GAPDH binds to cytokine mRNA•Changes in available GAPDH regulate cytokine mRNA translation
Contrary to previous understanding, activated T cells switch from oxidative phosphorylation to aerobic glycolysis not to promote proliferation but instead to augment the production of the antimicrobial protein IFN-γ, which is regulated by the glycolytic enzyme GAPDH.
Activated effector T (TE) cells augment anabolic pathways of metabolism, such as aerobic glycolysis, while memory T (TM) cells engage catabolic pathways, like fatty acid oxidation (FAO). However, ...signals that drive these differences remain unclear. Mitochondria are metabolic organelles that actively transform their ultrastructure. Therefore, we questioned whether mitochondrial dynamics controls T cell metabolism. We show that TE cells have punctate mitochondria, while TM cells maintain fused networks. The fusion protein Opa1 is required for TM, but not TE cells after infection, and enforcing fusion in TE cells imposes TM cell characteristics and enhances antitumor function. Our data suggest that, by altering cristae morphology, fusion in TM cells configures electron transport chain (ETC) complex associations favoring oxidative phosphorylation (OXPHOS) and FAO, while fission in TE cells leads to cristae expansion, reducing ETC efficiency and promoting aerobic glycolysis. Thus, mitochondrial remodeling is a signaling mechanism that instructs T cell metabolic programming.
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•TM cells have fused mitochondria while TE cells have fissed mitochondria•Mitochondrial fusion protein Opa1 is required for TM cells and not TE cells•Enforcing fusion improves adoptive cellular immunotherapy against tumors•Cristae remodeling via fusion/fission signals metabolic adaptations in T cells
The fate of T cells is in the shape of their mitochondria: remodeling mitochondrial cristae via fusion/fission instructs metabolic adaptations in T cells and can be modulated to enhance antitumor immunity.
CD8+ T cells undergo major metabolic changes upon activation, but how metabolism influences the establishment of long-lived memory T cells after infection remains a key question. We have shown here ...that CD8+ memory T cells, but not CD8+ T effector (Teff) cells, possessed substantial mitochondrial spare respiratory capacity (SRC). SRC is the extra capacity available in cells to produce energy in response to increased stress or work and as such is associated with cellular survival. We found that interleukin-15 (IL-15), a cytokine critical for CD8+ memory T cells, regulated SRC and oxidative metabolism by promoting mitochondrial biogenesis and expression of carnitine palmitoyl transferase (CPT1a), a metabolic enzyme that controls the rate-limiting step to mitochondrial fatty acid oxidation (FAO). These results show how cytokines control the bioenergetic stability of memory T cells after infection by regulating mitochondrial metabolism.
► CD8+ memory T cells possess substantial mitochondrial spare respiratory capacity (SRC) ► IL-15 regulates SRC by promoting mitochondrial biogenesis and CPT1a expression ► SRC in CD8+ memory T cells is dependent on mitochondrial fatty acid oxidation (FAO) ► Mitochondrial FAO enhances T cell survival and promotes CD8+ memory T cell development
Generation of CD8+ memory T cells requires metabolic reprogramming that is characterized by enhanced mitochondrial fatty-acid oxidation (FAO). However, where the fatty acids (FA) that fuel this ...process come from remains unclear. While CD8+ memory T cells engage FAO to a greater extent, we found that they acquired substantially fewer long-chain FA from their external environment than CD8+ effector T (Teff) cells. Rather than using extracellular FA directly, memory T cells used extracellular glucose to support FAO and oxidative phosphorylation (OXPHOS), suggesting that lipids must be synthesized to generate the substrates needed for FAO. We have demonstrated that memory T cells rely on cell intrinsic expression of the lysosomal hydrolase LAL (lysosomal acid lipase) to mobilize FA for FAO and memory T cell development. Our observations link LAL to metabolic reprogramming in lymphocytes and show that cell intrinsic lipolysis is deterministic for memory T cell fate.
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•Unlike Teff cells, memory T cells do not acquire substantial amounts of long-chain FA•Glucose supports mitochondrial FAO and OXPHOS in memory T cells•Memory T cells use LAL-mediated cell-intrinsic lipolysis to mobilize FA for FAO•T cell-intrinsic lysosomal lipolysis is important for memory T cell development
CD8+ memory T cells engage fatty-acid oxidation (FAO); however, the source of fatty acids that fuel FAO is unclear. O’Sullivan et al. show that memory T cells rely on glucose, and cell-intrinsic lipolysis to mobilize substrates, for FAO.
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